1. Field of Invention
Compressed-air spraying as a free-jet air blast is a reliable method for surface treatment with spray material. The method requires a compressor as a source of compressed air, a compressed-air drier, a compressed-air filter, a spray medium container for metered supply of the spray medium, and a hose line having a nozzle, which is generally a Laval nozzle. The performance or efficiency of the method is determined by the parametric field of the air-delivery volume of the compressor as a function of the necessary final pressure, spray medium flow rate, hose length, pressure ahead of the nozzle, and nozzle size.
When it is a question of cleaning and roughening surfaces with only a single application of the spray medium, typical operating values under normal conditions are: 8 mm nozzle diameter; 250 mm spacing from the surface of the spray material; and 80 mm spray-spot diameter, corresponding to approximately 5000 mm.sup.2 spray surface. The rate of flow of spray medium depends upon the required surface quality. Naturally, less spray medium is required for cleaning than for attaining a clean metallic surface having a certain peak-to-valley height.
Experience has shown that the effectiveness, i.e., the capacity for doing work, of the spray medium flow decreases very rapidly along the path between the nozzle outlet and the surface to be treated since the supersonic-flow rate is decreased very rapidly to the subsonic range. The theoretically most favorable working distance from zero cannot be realized in practice, since the spray spot cannot be permitted to fall below a critical small surface. This, however, corresponds to a working distance over the length of which the undesired spray speed reduction already occurs.
2. Description of the Prior Art
The foregoing drawback is even more pronounced when working in or below water. Still further drawbacks are as follows:
1. After acceleration in the Laval nozzle, the spray medium enters a medium which in many cases has a higher density. Consequently, the accelerated spray medium very quickly decreases in speed, so that the spray medium has hardly any effect when it encounters the surface to be treated if a water gap exists between the nozzle outlet and the surface to be treated.
2. It is only possible to perform work by a slanted application of the Laval nozzle directly on the surface; in this way the spray spot diameter equals the nozzle diameter. For an 8 mm nozzle, the spray surface under water is only approximately 50 mm.sup.2, and a selected surface quality having a certain peak-to-valley height cannot be attained under these conditions.
3. A counterpressure which increases proportionally to depth of application occurs in the Laval nozzle.
The foregoing drawbacks result not only when treating a surface with the cleaning spray medium, but also when treating surfaces with protective or coating spray medium.